The use of and a process for making novel epoxidized monohydroxylated polydienes in UV curable adhesive and sealant and compositions is described in U.S. Pat. No. 5,776,998. The polymers are combined with other ingredients such as a tackifying resin to make them suitable for adhesive and sealant products. A photoinitiator is included in the combination to promote UV curing (crosslinking) of the composition. A prior art method for making such materials involved blending the components in a nonaqueous solvent such as tetrahydrofuran (THF) and then casting adhesive films from the solution. THF was used because the particular commercial photoinitiator (mixed triaryl sulfonium hexafluoroantimonate salts in propylene carbonate) selected was not otherwise soluble in the adhesive formulation but was soluble in THF. After the adhesives were solvent cast, the THF was evaporated to leave the photoinitiator adequately dispersed in the remaining adhesive film to initiate effective UV curing upon exposure to UV light.
For many applications, the use of nonaqueous solvents is undesirable because of environmental hazards and the cost of nonaqueous solvent removal and a nonaqueous solvent itself. U.S. Pat. No. 5,776,998 described a nonaqueous solvent-free process for dispersing the same and other photoinitiators in an epoxidized monohydroxylated diene polymer formulation so that an effectively cured adhesive or sealant is produced without the problems of the solvent. The process involved mixing the components without a nonaqueous solvent under high shear conditions in a high shear mixer at a shear rate of at least 38,000 S−1 or in a sonicator at a power density of at least 4 watts per milliliter.
The process of U.S. Pat. No. 5,776,998 was able to achieve fine dispersions having droplet sizes of less than 10 micron diameter as described in the patent. In fact, the best dispersions produced by the method of the patent were able to achieve dispersions having a droplet size of approximately one-half micron diameter. As described in the patent, good UV cured adhesive and sealant products could be produced with the dispersions produced by the mixing method described in the patent.
Experience with these dispersions (emulsions) has shown that it is very important to have a very small particle size photoinitiator emulsion. Those photoinitiator emulsions that appear by to be about 0.5 micron diameter are much more beneficial than emulsions at 1 or more micron diameter. The very fine emulsions allow almost instant curing when used at levels as low as 0.04 to 0.10 weight percent (wt %) active initiator based on the entire adhesive formulation. In addition to the obvious advantage of needing less of what is the most expensive component of the formulation, the very low levels allow the final cured formulation to have the greatest degree of heat stability. The lower the amount of residual acid, generated by the UV exposure of the photoinitiator, the less bond breakage occurs in the gel network.
A preferred method of introducing the photoinitiator emulsion to the formulation has been to first make a concentrated emulsion of the photoinitiator in one of the components of the formulation, and then add it as the last component of the formulation. Pre-making the photoinitiator concentrate has several advantages. First, it allows the photointiator emulsion to checked before use. Second, it allows the emulsion to be prepared at another site where the needed equipment is located. Third, it allows a much smaller amount of material be processed on the sonifier or very high shear equipment, thus reducing equipment size. Fourth, it allows an accurate amount of photoinitiator to be easily added to the formulation. Most often the photoinitiator concentrate has been a 5 wt % emulsion of tri-aryl sulfonium hexafluoroantimonate salt in KRATON LIQUID® Polymer L-1203 (mono-ol polymer) prepared by sonication. Sometimes it has been a 5 wt % emulsion of the same salt in KRATON LIQUID® Polymer L-207 (epoxidized mono-ol polymer) prepared by sonication.
Several improvements in the photoinitiator concentrate and its production have become apparent from working with industry. First, there has been a strong need to be able to produce the emulsion on lower shear, more readily available equipment than a sonicator or very high shear rotor/stator equipment. Second, when the photoinitiator emulsion concentrate is made ahead of time or at another location, an important feature of the emulsion concentrate is its shelf stability under storage and shipping conditions. It is very important to make dispersions which have the least tendency to agglomerate and increase in particle size, for the reasons stated above. Increased stability has been greatly desired. Third, there has always been the need to make the smallest droplet size emulsion to extract maximum cure for the minimum usage.